Polycarbonates are used for many purposes (such as spectacles lenses and window glass) in which strength, heat resistance and transparency are required as they have excellent impact resistance, a high heat distortion temperature and high transparency. Although polycarbonates which are most widely produced are homopolymers formed by polymerizing bisphenol A (BPA), the development of a polycarbonate which is more excellent in performance is desired along with the expansion of their application. To cope with the expanding application, researches into copolymers obtained by introducing various comonomer units into commonly used monomer raw materials such as BPA are now under way. It is known through the researches into copolymers that a polycarbonate-polydiorganosiloxane copolymer comprising BPA and polydiorganosiloxane is superior to BPA homopolycarbonates in flame retardancy and low-temperature impact resistance (Patent Documents 1 to 3).
However, it is considered that it is difficult to produce a transparent (that is, high total light transmittance and low haze) polycarbonate-polydiorganosiloxane copolymer. The polycarbonate-polydiorganosiloxane copolymer is generally produced by reacting a mixture of a polydiorganosiloxane-containing bisphenol and a dihydric phenol such as BPA with an aqueous acid receptor such as phosgene or a sodium hydroxide aqueous solution under an interfacial condition (Patent Documents 3 to 6). The polycarbonate-polydiorganosiloxane copolymer obtained by this production process is clouded and opaque.
Attempts to produce a transparent polycarbonate-polydiorganosiloxane copolymer are described in Patent Documents 7 to 11. However, the transparency of the polycarbonate-polydiorganosiloxane copolymer obtained by this production process cannot reach a high level which is required for transparent resins.
Meanwhile, Patent Document 12 proposes a resin composition which is obtained by mixing together a first polycarbonate-polydiorganosiloxane copolymer with a second polycarbonate-polydiorganosiloxane copolymer and has polydiorganosiloxane domains having an average domain size of 20 to 45 nm buried in the matrix of a polycarbonate polymer. It is disclosed that this resin composition has impact resistance and a certain level of translucency. More specifically, it is disclosed that this resin composition has translucency (defined as having a light transmittance of about 25 to about 85% and a haze of less than about 104), can achieve an aesthetical visual effect when it is combined with a visual effect additive, is excellent in impact resistance and flame retardancy and has low weld-line visibility. However, this proposal does not disclose the relationship between the average size of the polydiorganosiloxane domains and transparency (light transmittance of not less than 85%).
Further, Patent Document 10 teaches that a polycarbonate-polydiorganosiloxane copolymer which provides a transparent molded article by an ordinary molding technique provides a clouded and opaque molded article when it is molded while it is retained in a high-temperature cylinder. That is, it is known that the transparency of the copolymer is readily affected by molding conditions. This patent document teaches that white turbidity can be suppressed by mixing an acid compound such as phosphorous acid (H3PO3). However, mixing of phosphorous acid exerts a bad influence such as the corrosion of production equipment on the resin composition and a molded article thereof. Therefore, a polycarbonate-polydiorganosiloxane copolymer which can maintain transparency even under the molding condition that it is retained in a high-temperature cylinder without mixing phosphorous acid has been desired.
To improve the flame retardancy of a polycarbonate resin, a resin dripping phenomenon must be suppressed. In a method in which an aromatic polycarbonate resin having a branched unit is used as a base resin, although transparency is excellent and a dripping improving effect is seen, a polycarbonate resin which is completely satisfactory in terms of both drip preventing effect and fluidity is not obtained yet.    (Patent Document 1) JP-A 5-186675    (Patent Document 2) JP-A 5-247195    (Patent Document 3) JP No. 2662310    (Patent Document 4) JP-A 3-79626    (Patent Document 5) JP-A 4-202466    (Patent Document 6) European Patent No. 0500087    (Patent Document 7) JP-A 6-100684    (Patent Document 8) JP-A 6-263865    (Patent Document 9) JP-A 8-169947    (Patent Document 10) JP-A 2006-518803    (Patent Document 11) JP-A 2005-535761    (Patent Document 12) JP-A 2006-523243    (Patent Document 13) JP-A 07-258532